The present invention is directed to high speed digital phase shifting apparatus and more particularly to digital frequency translation using the serrodyne principle.
Fast phase shifters are required in several areas particularly in the radar and Electronic Counter Measures (ECM) fields. In general, the phase shifters use a serrodyne system where swift phase changes result in nearly pure sideband generations. Serrodyning is a prevailing ECM technique used to prevent continuous wave (CW) or doppler tracking radars from gaining accurate velocity information. Such a system is shown in co-pending application, Ser. No. 534,566, filed Sep. 22, 1983, in the names of Asad M. Madni and Joseph Fala (Asad M. Madni is one of the co-inventors of the present application), entitled: "VELOCITY DETECTION APPARATUS". The disclosure of the Madni/Fala application is hereby incorporated by reference.
That application discloses a digital phase-shifter which produces a slowly changing false doppler frequencies. Thus, it performs a velocity gate stealer (VGS) function. VGS pulls the velocity tracker off the target return and drops it. The radar may then lock on to clutter or be forced into a reacquisition sequence. Typical frequency translations for the VGS function are between 20 Hz and 200 KHz which, for a 32-step operation implies clock frequencies of 6.4 MHz. Since the phase shifter must switch at least twice this rate, this implies a necessary switching frequency of 12.8 MHz or switching time of less than 20 nsecs. As disclosed in the above co-pending application, the switching time is met by, for example, utilizing a digital phase-shifter of the Schiffman type where each cell of the phase-shifter is driven by an output of a multi-bit counter whose clock input in turn is driven by a voltage-to-frequency converter which has a pulse train output. Each cell of the digital phase-shifter is sequentially switched in and out of the phase-shifter circuit by diode switches as disclosed in the above-mentioned pending application. Such diode switches may typically be of the PIN type (meaning P and N semi-conductor materials separated by an intrinsic layer). Typical switching speeds of PIN diodes are 10 nsecs with 2 to 3 nsecs achievable in some applications. Thus, PIN diode switches are very adequate for the foregoing requirement.
Another type of phase-shifter which can be utilized is a varactor phase-shifter. However, this requires a digital to analog converter. In combination with the limitations of the above conversion and the non-linearities in the varactor diode itself, a varactor type phase-shifter suffers from poor accuracy, complexity and limited speed. In contrast a digital computer can drive a fully digital phase-shifter directly.
In addition to doppler and CW radar, there is a pulse compression radar which goes under the acronym CHIRP. Here, a gate stealer ECM technique can be generated in exactly the same manner as a velocity gate stealing technique, in that a range tracker is pulled off the true target return and then dropped. This, however, with use of a digital phase-shifter implies switching times in the order of pico-seconds. Such short times are not achievable using PIN diodes as switching elements.